Technical Field
The present invention relates to a switching element driving circuit that includes an overcurrent protection function for such switching elements as IGBT (insulated-gate bipolar transistor) and MOS-FET (metal-oxide-semiconductor field-effect transistor).
Background Art
A well-known power converter to drive AC load is a power converter having two switching elements Q1 and Q2, which are totem pole-connected to form a half bridge circuit, and alternately turn ON/OFF so as to switch the DC voltage. The switching elements Q1 and Q2 are constituted by a high voltage IGBT or a high voltage MOS-FET, for example. It is also known that an overcurrent protection circuit for the switching elements Q1 and Q2 is built into a switching element driving circuit that drives the switching elements Q1 and Q2 ON/OFF.
FIG. 6 is a schematic block diagram depicting a key section of an example of a conventional switching element driving circuit, where Q1 to Q6 are six switching elements constituted by IGBTs which form a half bridge circuit for each phase (U phase, V phase and W phase) of a three-phase AC load. The switching element driving circuit 1 is disposed for each of the switching elements Q1 to Q6, and drives each switching element Q1 to Q6 ON/OFF correlating with each other under a predetermined phase relationship.
In FIG. 6, the switching element driving circuit 1 for the switching element Q2 is illustrated, but the switching element driving circuits for the other switching elements Q1 and Q3 to Q6 also have the same configuration.
Here the switching element driving circuit 1 includes a driving circuit main unit 2 that applies predetermined gate voltage VG to the switching element Q2, to drive the switching element Q2 ON/OFF. This driving circuit main unit 2 is constituted by a p-channel MOS-FET and an n-channel MOS-FET which are totem pole-connected, for example. The p-channel MOS-FET and the n-channel MOS-FET complementarily turn ON/OFF when a driving control signal CS is received, and generate the gate voltage VG as pulses.
An overcurrent protection circuit 3, included in the switching element driving circuit 1, has a comparator CMP that detects the current flowing through the switching element Q2 by converting the current into voltage via voltage dividing resistors R1 and R2, and compares the detected voltage Vi and a reference voltage Vb1 to specify the overcurrent threshold. The comparator CMP plays a role of disabling input of the driving control signal CS to the p-channel MOS-FET by setting a flip-flop FF when overcurrent is detected. Therefore when the flip-flop FF is set, driving the switching element Q2 ON by the driving circuit main unit 2 is stopped.
The overcurrent protection circuit 3 includes a differential amplifier AMP that controls the gate voltage VG applied to the switching element Q2 based on the reference voltage Vb2, which is set higher than the reference voltage Vb1 (Vb2>Vb1), and the detected voltage Vi. This differential amplifier AMP plays a role of releasing the switching element Q2 from the overcurrent state by dropping the gate voltage VG down to the reference voltage Vb2 when the detected voltage Vi is higher than the reference voltage Vb2. The function to control the current flowing through the switching element Q2 by the differential amplifier AMP is described in detail, for example, in Japanese Patent Application Laid-Open No. 2010-62860.